Reducing Fossil Fuel Power Station Pollution: Strategies And Innovations

Burning fossil fuels creates emissions of harmful pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOX), particulate matter (PM), carbon dioxide (CO2), and mercury (Hg), which have negative effects on both the environment and human health. Fossil fuel-fired power plants are a leading source of air, water, and land pollution, with over 3,400 such power plants in the US alone. These plants are the largest stationary source of nitrogen oxide and sulfur dioxide emissions and a significant source of mercury and fine particle emissions. They also contribute to carbon dioxide emissions, which have a substantial impact on climate change. To reduce pollution from fossil fuel power stations, regulatory bodies such as the US Environmental Protection Agency (EPA) have proposed and implemented various standards and initiatives, including the Clean Air Act, to limit emissions and improve public health.

Using low-sulfur-content coal to reduce sulfur dioxide emissions

Burning low-sulfur-content coal is one of the ways to reduce sulfur dioxide emissions from fossil fuel power stations. Sulfur dioxide (SO2) is a harmful pollutant that contributes to acid rain, respiratory illnesses, and heart disease, particularly in children and the elderly. It is released during the combustion of fossil fuels, such as coal, which naturally contains sulfur.

To reduce SO2 emissions, power plants can switch to burning coal with a lower sulfur content. This approach directly targets the source of SO2 by reducing the amount of sulfur available for release during combustion. The coal industry has found ways to reduce sulfur and other impurities from coal, and some coal consumers have already made the switch to low-sulfur coal.

However, the use of low-sulfur coal has its limitations. For instance, this method is not efficient and has limited global application. The majority of sulfur oxide (SOx) emissions in the air come from coal-fired power plants, and the effectiveness of SOx reduction depends on both the sulfur content in the coal and the emission control system used.

Other methods to reduce SO2 emissions include the use of flue gas desulfurization (FGD) systems, which employ sorbents to scrub SOx from the flue gas. Additionally, power plants can cofiring wood chips with coal or pretreat and process coal to further reduce undesirable compounds in combustion gases.

Cofiring wood chips with coal to reduce sulfur dioxide emissions

Burning fossil fuels like coal produces harmful substances such as carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, and heavy metals like mercury. These byproducts have detrimental effects on both the environment and human health.

To reduce air pollution, power plants can adopt various methods, such as burning low-sulfur-content coal, cofiring wood chips with coal, or pretreating and processing coal before combustion. One effective method to reduce sulfur dioxide (SO2) emissions is by cofiring wood chips with coal.

Cofiring wood chips with coal is a process where wood chips are burned alongside coal in power plants to reduce SO2 emissions. This method has been proven to decrease SOx emissions beyond what would be expected from simply replacing sulfur-bearing coal with natural gas. The enhanced reduction of sulfur emissions, known as sulfur leveraging, occurs due to increased sulfur capture in coal ash. Sulfur leveraging is influenced by factors such as residence time and furnace temperature, with longer residence times and higher temperatures resulting in greater SO2 reduction.

The primary mechanism behind sulfur leveraging is the conversion of SO2 to a more reactive form, SO3, within the natural gas flame. This process increases the reactivity of the ash sorbent, leading to improved sulfur retention in the ash. Additionally, cofiring with natural gas has been shown to be a cost-effective method for reducing emissions, with all cofiring policies reducing CO2 emissions at less than $13/ton.

By implementing a cofiring standard, power plants can significantly reduce their emissions. A plant-specific rate-based standard, for example, would require each plant to lower its emissions rate, allowing for cofiring with natural gas or the installation of other emission-reducing technologies. This approach ensures that even the least flexible policies result in substantial emissions reductions.

Using bag-houses to trap particulates

Bag-houses are large filters that can be used to trap particulates and reduce pollution from fossil fuel power stations. They are also known as baghouse filters, bag filters, or fabric filters. They are an air pollution control device and dust collector that removes particulates or gas released from commercial processes out of the air.

Bag-houses came into widespread use in the late 1970s after the invention of high-temperature fabrics capable of withstanding temperatures over 350 °F (177 °C). They are commonly used by power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers, and other industrial companies to control emissions of air pollutants.

Baghouses typically use long, cylindrical bags (or tubes) made of woven or felted fabric as a filter medium. The dust-laden gas or air enters the baghouse through hoppers and is directed into the baghouse compartment. The gas is drawn through the bags, and a layer of dust accumulates on the filter media surface until air can no longer move through it. When a sufficient pressure drop occurs, the cleaning process begins.

Baghouses are very efficient particulate collectors due to the dust cake formed on the surface of the bags. The fabric provides a surface on which dust collects through the following four mechanisms:

• Inertial collection: Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
• Interception: Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.
• Brownian movement: Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
• Electrostatic forces: The presence of an electrostatic charge on the particles and the filter can increase dust capture.

The combination of these mechanisms results in the formation of a dust cake on the filter, which eventually increases resistance to gas flow. The filter must be cleaned periodically. To ensure a long lifespan, the filter bags are commonly coated with a filter enhancer (pre-coat). The use of chemically inert limestone (calcium carbonate) is most common as it increases the efficiency of dust collection (including fly ash) via the formation of a dust cake or coating on the surface of the filter media.

There are three common types of baghouses, classified by their cleaning method: mechanical shakers, reverse gas, and pulse jet. Mechanical-shaker baghouses use vibration to create waves in the bags to shake off the dust cake. Reverse-air baghouses clean the bags by injecting clean air into the dust collector in a reverse direction, causing the bags to collapse partially and the dust cake to crack and fall off. Pulse-jet baghouses, on the other hand, use a short burst of compressed air injected through a common manifold over a row of bags to flex the bags and break the dust cake.

Using electrostatic precipitators to remove particulates

Electrostatic precipitators are large industrial emission control units used to remove dust and fine particles from exhausted gases. They are highly effective at reducing particle pollution and can handle large volumes of gas at various temperatures and flow rates.

The first step in the electrostatic precipitation process is to charge the particles in the gas stream. This is achieved by applying a high voltage direct current to the discharge electrode, creating a corona discharge. Ions and electrons are produced, and the charged particles are then attracted to and deposited on the collection electrode. When enough particles have accumulated, they are shaken off into a hopper by mechanical rappers. The particulates are then transported away for disposal or recycling.

The basic design of an electrostatic precipitator consists of a row of thin vertical wires and a stack of large flat vertical metal plates. The contaminated gas flows horizontally between the wires and then through the stack of plates. A negative voltage of several thousand volts is applied, ionizing the particles in the gas stream, which are then attracted to the grounded metal plates.

Electrostatic precipitators are highly effective at removing harmful particulate matter, including fine particles smaller than 2.5 microns in diameter, which can be drawn deep into the lungs and cause serious health issues such as lung damage and bronchitis. They are commonly used in fossil fuel power-generating stations and can capture up to 99.9% of dust particles.

The performance of electrostatic precipitators can be close to 99.5%, and they have a long life expectancy of over 20 years if properly maintained. They are energy-efficient as they only apply energy to the particulate matter, and their filtration performance is durable over time compared to other filters.

Overall, electrostatic precipitators are an important tool for reducing pollution from fossil fuel power stations by effectively removing particulates from exhausted gases.

Using wet scrubbers to remove PM from combustion gas

Wet scrubbers are an effective method for removing particulate matter (PM) from combustion gas in fossil fuel power stations. They are instruments that use liquid solvents, usually water, to absorb and remove pollution from gases.

In the context of fossil fuel power stations, the combustion gas enters the wet scrubber at the bottom and passes upward through a packed bed, while downward-flowing solvent sprays capture pollutants. The gas then passes through a mist eliminator to capture any remaining droplets before exiting the scrubber. This process is known as absorption, and it is highly effective, with most absorbers achieving removal efficiencies of over 90%.

The effectiveness of a wet scrubber depends on several factors, including the solvent's composition and electric charge, the liquid flow rate, and the target gas velocity. The solvent's ability to absorb contaminants is critical, and its electric charge determines which inorganic pollutants it can bind to. Common solvents used in wet scrubbers include sodium hydroxide, calcium hydroxide, and sodium carbonate.

Wet scrubbers are vital in industrial settings, particularly in manufacturing and wastewater treatment facilities, as they prevent pollutants from escaping into the outside air. They are also advantageous due to their relatively low pressure drop, high mass-transfer efficiencies, low capital cost, and small space requirements.

To ensure the effective removal of PM, it is essential to treat the scrubbing liquid before discharging or reusing it to guarantee total pollutant removal. Additionally, the performance of a wet scrubber depends on the purity of the water used, and addressing secondary pollution is crucial for maintaining the system's efficiency.

Wet scrubbers are a proven technology for reducing PM emissions from fossil fuel power stations, and they play a vital role in minimizing environmental impact and protecting human health.

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